1. Field of the Invention
This invention relates to an IC socket in which a movable plate is movable in a horizontal direction by a pivotal operation of a crank lever, to thereby establish or release a contact condition or relationship between a contact on a socket body and a lead on an IC package.
2. Brief Description of the Prior Art
Laid-open (Kokai) Japanese Patent Application No. 278778/87 discloses an IC socket in which a movable plate having a number of lead insertion through-holes is placed on an upper surface of a socket body provided with contacts, and the movable plate is movable in a horizontal direction by a pivotal operation of a crank lever, to thereby establish or release a contact condition or relation between the contacts and IC leads. This IC socket is widely used as a socket for a pin grid array type IC which requires high inserting and removing loads.
The above crank lever is provided with an operating shaft portion and a crank shaft portion which are disposed perpendicular with respect to each other. As shown in FIGS. 1(A), 1(B), and 1(C), the crank shaft portion 3a is disposed between the socket body 1 and the movable plate 2. The crank shaft portion 3a is rotated about a fixed fulcrum on a side of the socket body 1 by a pivotal operation of the operating shaft portion and at the same time, the eccentric shaft portion 3b is eccentrically pivoted about the fixed fulcrum of the crank shaft portion 3a on the side of the movable plate 2. This eccentric pivotal force is applied to the movable plate 2 in order to move it forwardly or backwardly along the upper surface of the socket body 1 depending on the pivotal direction of the eccentric shaft portion 3b. Upon a forward movement of the movable plate 2, the IC lead, which had been inserted between contact pieces of each of the contacts through a corresponding lead insertion through-hole formed in the upper surface of the movable plate 2, is retracted therefrom, thus enabling a no-load insertion and removal. Furthermore, the lead is inserted between the contact pieces upon a backward movement of the movable plate.
Accordingly, the eccentric shaft portion 3b is effective to establish a contact condition or relation between the IC lead and the contact moved to a starting point P.sub.1 of its pivotal movement, and to remove the contact condition or relation therebetween when moved to a terminal point P.sub.2 of its pivotal movement.
In the above prior art, in order to achieve the contact or contact release condition, the starting point P.sub.1 of the pivotal movement of the eccentric shaft portion 3b is set in a second or third quadrant of the coordinates which are formed by an X-axis and a Y-axis passing through a central point of the pivotal movement of the crank shaft portion 3a, and the eccentric shaft portion 3b is eccentrically pivoted clockwise from the starting point P.sub.1 of the pivotal movement set in the second quadrant towards the terminal point P.sub.2 of the pivotal movement set in the first quadrant. Alternatively, the eccentric shaft portion 3b is eccentrically pivoted counterclockwise from the starting point P.sub.1 of the pivotal movement set in the first quadrant towards the terminal point P.sub.2 of the pivotal movement set in the second quadrant.
Whether the starting point P.sub.1 of the pivotal movement is set in the second quadrant or to the first quadrant depends on whether the crank shaft portion 3a is disposed at one side of the socket body 1 and movable plate 2 or the other side thereof.
As discussed above, the eccentric shaft portion 3b has the starting point P.sub.1 of the pivotal movement set in the second or first quadrant about the crank shaft portion, and the contact condition between the contact and the IC lead is established when the eccentric shaft portion 3b is positioned at this starting point P.sub.1 of the pivotal movement.
When an external force W attempting to move the movable plate 2 forwardly is applied thereto when the eccentric shaft portion 3b is positioned at the starting point P.sub.1 of the pivotal movement in the second or first quadrant, component forces W.sub.1 and W.sub.2 directed in a horizontal direction along the X-axis and in an upward direction along the Y-axis as shown in FIGS. 1(A) and 2, act against the eccentric shaft portion 3b. These component forces W.sub.1, W.sub.2 positively act to urge the movable plate 2 in the forward direction. As a result, the contact condition of the IC is likely to be removed. It will be understood that this unfavorable phenomenon occurs because of the fact that the starting point P.sub.1 of the pivotal movement of the eccentric shaft portion 3b is located above the X-axis which passes through the center of the crank shaft portion 3a (i.e., in the second or first quadrant).
As one attempt for preventing this unfavorable phenomenon, heretofore, the movable plate 2 has been provided with a braking projection projecting from a side surface of the movable plate 2 to engage with an operating shaft portion of the crank lever. However, because this projection must be tiny so that the operating shaft portion can easily climb over it, the crank lever accidentally climbs over this tiny projection when an external force applied to the movable plate is large, with the result that a contact release condition is undesirably established.
The above situation becomes even worse when the operating lever is operated to move between its vertical position and its horizontal position repeatedly, because the tiny projection becomes worn. Moreover, when an external force acting on the movable plate in an attempt to move it forwardly is excessively large, the tiny projection can be broken. As a result, the function of the projection in holding the crank lever at the starting point (i.e., contact position) of the pivotal movement is lost. This problem is also true when the eccentric shaft portion is located on the X-axis.
As a result of use of the prior art device for many years, the above problems became manifest. The present invention is made in order to solve the above problems.